The present invention relates to an ink jet head and an ink jet recording apparatus. Specifically, the present invention relates to an ink jet head for ejecting ink as ink droplets, and to an ink jet recording apparatus using the ink jet head, or the present invention relates to an electrostatic ink jet head for ejecting ink droplets by exerting an electrostatic force onto ink at a tip end of an ink guide, and to an ink jet recording apparatus using the electrostatic ink jet head.
As an ink jet recording system in which ink is ejected as ink droplets, there are an electrostatic system in which ink droplets are ejected by exerting an electrostatic force onto the ink, an electrothermal conversion system in which ink droplets are ejected by pressure of steam generated by heat of a heating element, a piezoelectric system in which ink droplets are ejected by mechanical pressure pulses generated by a piezoelectric element, and the like.
As the electrostatic ink jet recording system in which ink is ejected toward a recording medium by using the electrostatic force, for example, there is a system in which ink containing charged fine particles is used, and ink ejection is controlled by utilizing electrostatic force through application of a predetermined voltage (drive voltage) to ejection electrodes (drive electrodes) of an ink jet head in correspondence with image data to thereby record an image corresponding to the image data on a recording medium. For example, an ink jet recording apparatus disclosed in JP 10-138493 A (hereinafter, referred to as Patent Document 1) is known as an apparatus using such electrostatic ink jet recording system.
The electrostatic ink jet recording apparatus disclosed in Patent Document 1 has a configuration in which an ink guide is disposed in a through hole functioning as a nozzle for ejecting the ink, and an ejection electrode is disposed in a periphery of the through hole. The ink jet recording apparatus generates electric fields around the through holes through application of voltages to the ejection electrodes corresponding to recording data, causing the force from the electric fields act on the meniscuses of the ink formed at the tip ends of the ink guides, and ejects the ink droplets from the tip ends of the ink guides to a recording medium. The ink jet head according to the electrostatic ink jet system is capable of forming minute droplets and has a simple structure, and therefore has an advantage in that it is easy to form a multichannel structure in which a plurality of ejection ports (channels) are arrayed on one head.
FIG. 20 is a schematic configuration view of an example of the ink jet head of the electrostatic ink jet recording apparatus disclosed in Patent Document 1. In an ink jet head 300 illustrated in FIG. 20, only one ejection portion of the ink jet head disclosed in Patent Document 1 is conceptually illustrated. The ink jet head 300 comprises a head substrate 302, an ink guide 304, an insulating substrate (i.e., ejection port substrate) 306, a control electrode (i.e., ejection electrode) 308, a counter electrode 310, a DC bias voltage source 312, and a pulse voltage source 314.
The ink guide 304 is disposed on the head substrate 302, and a through hole (i.e., ejection port) 316 is opened through the insulating substrate 306 at a position corresponding to the ink guide 304. The ink guide 304 extends through the through hole 316, and a convex tip end portion 304a thereof protrudes above the surface of the insulating substrate 306 on a recording medium P side. The head substrate 302 and the insulating substrate 306 are arranged to have a predetermined gap therebetween to form a flow path 318 of ink Q.
The control electrode 308 is arranged in a ring shape so as to surround the through hole 316 on the surface of the insulating substrate 306 on the recording medium P side for each ejection portion. The control electrode 308 is connected to the pulse voltage source 314 which generates a pulse voltage according to the image data, and the pulse voltage source 314 is grounded through the DC bias voltage source 312.
The counter electrode 310 is arranged at a position opposing the tip end portion 304a of the ink guide 304, and is grounded. The recording medium P is disposed on the surface of the counter electrode 310 on the ink guide 304 side. That is, the counter electrode 310 functions as a platen for supporting the recording medium P.
Upon recording, the ink Q containing fine particles (colorant particles) charged to the same polarity as that of the voltage to be applied to the control electrode 308 is circulated by a not shown ink circulation mechanism in a direction from the right side to the left side in the ink flow path 318 in FIG. 20. Further, a high voltage of, for example, 1.5 kV is always applied to the control electrode 308 by the DC bias voltage source 312. At this time, a part of the ink Q in the ink flow path 318 passes through the through hole 316 in the insulating substrate 306 due to a capillary phenomenon or the like, and is concentrated at the tip end portion 304a of the ink guide 304.
When the pulse voltage source 314 supplies the control electrode 308 biased to 1.5 kV by the bias voltage source 312 with a pulse voltage of, for example, 0V, the voltage of 1.5 kV obtained by superimposition of the pulse voltage on the bias voltage is applied to the control electrode 308. In this state, the electric field strength near the tip end portion 304a of the ink guide 304 is relatively low, so that the ink Q containing colorant particles which are concentrated at the tip end portion 304a of the ink guide 304 is not ejected from the tip end portion 304a of the ink guide 304.
On the other hand, when the pulse voltage source 314 supplies a pulse voltage of, for example, 500V, to the control electrode 308 which is biased to 1.5 kV, the voltage of 2 kV obtained by superimposition of the pulse voltage on the bias voltage is applied to the control electrode 308. Consequently, the ink Q containing colorant particles which are concentrated at the tip end portion 304a of the ink guide 304 is ejected as ink droplets R from the tip end portion 304a due to the electrostatic force, and is attracted to the grounded counter electrode 310 to adhere to the recording medium P, thereby forming dots of colorant particles.
In this way, recording is performed with dots of colorant particles while relatively moving the ink jet head 300 and the recording medium P supported on the counter electrode 310, thereby recording an image corresponding to the image data on the recording medium P.